mfd: pm8xxx-irq: fix rogue interrupt warnings

The current mask and mask_ack callbacks assume that if config entry
for an interrupt is 0, then it must be a rogue interrupt.
This is not true because an interrupt number that is a multiple of 8
and is a RISING and FALLING edge triggered, will have a config entry
of 0. The driver incorrectly identifies such an interrupt as a rogue
interrupt and prints a warning.

Fix this by using the PM_IRQF_WRITE flag to indicate a requested
interrupt.

Change-Id: I187276a19a4f74712498a7c799dba27858ca784b
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>